Signal sensing circuit

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squonk
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Signal sensing circuit

Post by squonk » Wed Oct 08, 2003 8:59 pm

Still trying to get the overhead paging system right. This time I need a circuit that will sense the incoming signal from the PBX console and pull in a relay that will apply the input to the paging amplifier. The signal comes in at line level, which is 1v peak to peak, I think. I want it to sense the click when the operator first “punches in.” is there a simple way to do this? Thanks in advance for any help.

dyarker
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Re: Signal sensing circuit

Post by dyarker » Thu Oct 09, 2003 6:03 am

Use a loop-start trunk card instead of a line card on the PBX for the paging system. A CO trunk will also work. Lines or trunks from PBXs are 600 Ohm balanced, not 1 Volt line. (A 600 Ohm to 10KOhm transformer can fix that.) Current through an opto-coupler LED to sense selected.<p>If, on the other hand, your PBX does have a line-out jack for a PA system, then there is almost certainly a set of relay contacts that close when PA is "dialed". Either near the jack, or on the frame. Norstar key systems send them to the frame.<p>Years ago I worked on a system the connected to a PBX line. It answered on ringing like an answering machine, but sound went to the speaker amplifier instead of tape.<p>Cheers,
Dale Y

squonk
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Re: Signal sensing circuit

Post by squonk » Thu Oct 09, 2003 3:55 pm

Some more info. This is a Convayent PBX. There is a relay connected somehow to a 50 pair punch down. I can hear the relay click but I can't find the contacts on the punch down. I don't have any experience with the circuit components you describe above. I will be working on the system tonight. Any more insight you can give will be greatly appreciated. Thanks again,<p>Bill

dyarker
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Re: Signal sensing circuit

Post by dyarker » Thu Oct 09, 2003 11:35 pm

Did you spell "Convayent" correctly? A search on Google resulted in ZERO hits. It's possible the company doesn't have a web site, but very unlikely that NOBODY is talking ABOUT them. So much for just looking it up and giving you an exact answer.<p>And, since you're asking here, I assume the manual is missing.<p>The PBX may be putting a DC voltage on the audio pair, or the relay contacts are probably one pair before or after the audio pair.<p>To test, pick an unused pair on the block ('cause nobody hooked-up PA before, right?) and put a voltmeter on the pair. Dial PA. If voltage changes, something to zero, or zero to something, you've found it. If the voltage is steady zero, change the meter to Ohms, dial PA again, if the resistance changes, open to short or short to open, you've found it. If resistance doesn't change, SET METER BACK TO VOLTS, and try another pair.<p>Some terminology. Trunk - a switch to switch voice path. CO - short for Central Office, a phone company switch. Line - goes from a switch to a phone.<p>From a PBX point of view, a phone company line is a trunk. If you find the manual it will call them trunks, CO trunks, CO lines, just CO, etc. The lines from the PBX will be called lines, subscribers, or drops.<p>It's hard to guess what they call hardware features like PA, audio in (for music-on-hold), external ringers, remote door locks, etc. But, on your 50 pair block, they won't be randomly mixed. The pairs for trunks will be together, pairs for lines together, and pairs for hardware features together.<p>That's it for now, unless you can help me find the tech data.<p>Cheers,
Dale Y

squonk
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Re: Signal sensing circuit

Post by squonk » Sat Oct 11, 2003 7:48 pm

Thanks, Dale, for your advice<p>Sorry! It’s conveyant.<p>I appreciate all the trouble you went to to help me! I found where the relay contacts punch down Thursday night. This is in a very busy hospital, which makes it very inconvenient to work on because it can never be shut down except for short periods late at night to early morning. Anyway, I haven’t got it hooked up the way I want it yet but feel confident I can now.
Now then, I am still interested in adding a backup microphone that does not go through the pbx . I would like to hook up a 600 ohm desk type microphone that goes strait to the amplifier input. The paging amplifier is about 250 feet from the switchboard operators.
I am using shielded wire yet when I plug it in there is some 60hertz hum. This is where that circuit I was asking about earlier comes in. what I don’t know about is the opto-coupler LED. I have a transformer like you mentioned . Could you detail this circuit more for me? <p>Thanks again,<p>Bill

dyarker
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Re: Signal sensing circuit

Post by dyarker » Sat Oct 11, 2003 10:11 pm

250 ft. is a L O N G way to go for a microphone. Does the mic work, without hum, when connected close to paging amp? Is the PA input labeled "MIC"? You can't just parrallel the mic with the phone system line either.<p>What kind of plug on the mic? Three conductor with push-to-talk switch sharing one wire with the mic element? No push-to-talk? Then the sensor you're asking for is a "VOX" (voice operated transmit) circuit.<p>Phone lines are 600 Ohm balanced, with an audio level in the -10dbm to 0dbm range. The 600 Ohm mic is probably neutral, with audio level of -30dbm to -20dbm. You crank the volume on the amp to compensate, and get lots of hum. Since you need a pre-amp anyway, get/build one with a VOX operated relay.<p>What model is the PA amp?
Dale Y

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